To print an image, a print engine processor, referred to herein as a raster image processor, converts the image in a page description language or vector graphics format to a bit mapped image indicating a value to print at each picture element (pixel or pel) of the image. (Note that the terms “pixel” and “pel” are used throughout this specification in an interchangeable manner. Each term refers to one dot or one point of data in the complete image).
Each bit representing a pel that is “on” is converted to an electronic pulse. The printer includes a rotating drum comprised of a photo-conducting surface with a coating capable of holding an electrostatic charge. In one implementation, the drum surface is initially negatively charged. The electronic pulses generated from the raster data control a laser beam that positively charges selected areas on the surface of the drum. In this manner, the pulsing of the laser beam creates an electrostatic representation of the image on the drum surface. Typically, the laser operates on one “scan line” at a time where a scan line is a set of pels representing one line of the image. After the laser beam charges all active pels from one scan line of the raster data, the drum rotates so the laser beam can operate on the next scan line.
The drum surface, including the positively charged electrostatic representation of the image, then passes over negatively charged toner. The negatively charged toner is attracted to the positively charged areas of the drum that form the image. The paper, which is even more negatively charged that the drum, passes over the drum and attracts the toner, thereby transferring the image from the drum to the paper. The toner is then fused to the paper using a pair of heated rollers.
The above discussion describes the drum, the toner, the paper, and other components as having certain charges enabling the formation of the image on the paper. Analogous printing systems, well known to those skilled in the art, utilize functionally similar components that operate using opposite charges, but provide the same end result.
Many modern laser printers filter the bit map images using a look-up table to alter the pulses generated for each pel to accomplish certain filtering results. For instance, filters can be used to provide an economy mode where toner is reduced, remove jagged edges, improve image quality, improve print quality using known techniques referred to as ‘Print Quality Enhancement’ (PQE) or reduce the density of images. The subject matter of the present invention is primarily concerned with filtering for PQE purposes, but is applicable to filtering generally.
Typically, a laser printer will analyze a contiguous area of pel data and modify the laser output intensity values for one or more of the pels in the area. For example, a laser printer may compare the pel pattern in an area to one or more predefined patterns stored in a look-up table. If a match is detected between the pel pattern and a pattern in the lookup table, the printer may modify the laser output intensity normally used for an “on” pel. The printer might, for example, use a pulse width modulator to shorten the pulse width to reduce the electric charge the laser beam places on the drum or an amplitude modulator to reduce the peak amplitude of the pulse to reduce the electric charge the laser beam places on the drum. (The present invention is primarily concerned with the use of an amplitude modulator to control the electric charge). For instance, if the amplitude modulator reduces the amplitude associated with a particular pel to one-half of the peak amplitude, the laser will discharge the photoconductor in the area of the drum surface corresponding to the pel with only one-half of the available power. The reduction in laser power causes a corresponding reduction in the amount of discharge on the drum surface that, in turn, causes a change in the amount of toner that is ultimately transferred to the paper. The look up tables used by the amplitude modulator to filter pel data vary the pulse amplitude to attain PQE are well known to those skilled in the relevant arts. Pels printed using less than the full amplitude for PQE purposes may be referred to as “gray-scale” pels as they are printed somewhere between white and black. Techniques other than look-up tables may also be utilized for PQE purposes as long as such techniques provide for the modification of amplitude values in response to recognition of certain patterns of pel data.
A system utilizing amplitude modulation for PQE purposes must feed an amplitude modulator with information indicating the peak amplitude of the pulse to create for each pixel. This means that the amplitude modulator must be able to accept and process the input and create and deliver the required pulse at the proper amplitude to the laser within the time it takes the printer to print one pel. As modern, high-function printers continue to operate at faster and faster speeds, it becomes more difficult for the amplitude modulator to keep pace. The latest printers are capable of operating at speeds of 100 MHz or greater, which leads to a “Pel Time”, or the time needed by the printer to process one pixel, of 10 ns or less. Another way to measure the speed of a printer is called the “Video Data Rate”, which is the number of pels written on the drum by the printhead per second. As printer speeds continue to increase, it is becoming more and more difficult for amplitude modulators to function properly. Increases in speed are more difficult to achieve in the amplitude modulators because some minimum time is required to reset the amplitude modulator after each pulse.
For the above reasons, there is a need in the art for an improved technique for processing pels of print output for PQE in order to allow amplitude modulators to keep up with the increasing printer speeds without reducing print image quality.